Must I guide this 12 nights Thailand/Malaysia/Singapore combined tour?

The hotels you mentioned are not top luxury, but are very decent and should be comfortable, no problem. Overall your itenery looks very packed and rushed. You get to do a lot of things, always with something on. If you are from US, have better amusement parks. If you have gone to Disneyland and dreamworld in US, then bangkok dream world and Malaysia genting outdoor theme park will disappoint. It looks like to be good for someone who wants a quick tour of the area. You probably will have future trips on your own to Thailand, and a separate one to Malaysia and Singapore (combined). You will miss out on what the destination has to offer as it is too rushed. I don’t like to rush to so many countries and places over such a short time. Overall I don’t think I would recommend this as you don’t get to experience what this area has to offer. Consider Malaysia and Singapore combined...